Coating compound



www f W. J. BALDWIN COATING COMPOUND Filed July 14, 1943 Mardi 19, 1946.

` resistant case.

Patented Mar. 19, 1946 william J. nalawimclen cumuli, assign@ to- Crane Co., Chicago, Ill., a corporation of Illinois Application July i4, 194s, seria1'No.494,s9s (ci. msj-z2) This invention relatesgenerally to a coating 1 claim.

compound. More particularly it pertains to the type used in surface hardening ferrous alloys or the like and to such protectivecoatings applied to selected parts of an iron or steel article prior y to hardening in order to prevent those parts from being affected by thehardening process or similar heat treatment. The protective coating of this invention has also been known to prevent undesirable scale formations on steel articles A protective coating to be successful in the selective siliconizing of ferrous metals must therewhich have been subjected toa heat treating process.

More particularly, the coating compound of this invention applies to the surface-hardening process known as siliconizing, the term used to denoteA theimpregnation of ferrous alloys with silicon in order to produce a. corrosionand wear- It is the usual practice in siliconizing to place the parts to be treated in a rotary type of 'furnace where they are heated to an elevated temperature and while so heated, contacting them with a reagent formed by heating a silicon-bearing material in a current of chlorine or chloride vapor. The temperatures required for cementation of the silicon by this method are from 1800" to at least 1900 F.

In the surface-hardening of ferrous alloys it is frequently desirable to limit the hardening effect to those parts alone which are required to be hardened and to prevent certain adjacent orpreselected portions from being hardened so as to avoid the resultant brittleness thereof, as in the case of screw-threaded portions or other parts which may be desired soft or untreated for subsequent machining or for other reasons.

It is known that methods have been used and coatings supplied for selective hardening of steel articles where the hard case was produced by the process of carburizing or that of nitriding, but these methods or coating compounds are ineffectual in preventing the hardening of selected areas in the above described process of siliconizing, due to the peculiar problems and conditions involved. These may be listed as follows:

1. Rotating action of retort in which parts are processed with consequent abrading action taking place between ferrous articles to be siliconized land the silicon-bearing reagent, usually silicon carbide. v

2. Effect of hot chlorine gas.

3. Elevated temperatures required. from 1800" to 1900 F.

4. Ease with which the silicon may diffuse through the protective coating and penetrate into the surface of the steel part being treated.

fore have the following characteristics:

1. Adequate protection of the parts to whic it is applied.

2. Capable of being easily applied with a brush. 3. Not'subject to shrinking or cracking under the high temperatures encountered.

4. Not subject to melting and/or peeling o when heated. f i, f

- 5.- Resistant to abrasive action when rotated in contact with the silicon-bearing reagent.

6. Easily removable after treatment. 7.. Low materialandl operating-cost, that is, should require no heat pretreatment in order to fuse, but fusion should take place in the siliconizing furnace. n

I T he chief object of this invention therefore is to provide a coating compound for vuse inthe selective siliconizing of ferrous alloys which will have the properties necessary to meet the drastic conditions imposed'by the siliconizing process, which is easily applied with a brush, and which can readily be removed after treatment.

A further object of the invention is the provision of a protective coating which will prevent scale formation on the surface of ferrous alloys during heat treatment.

To the accomplishment of the foregoing and related ends, the invention, then, consists of the features hereinafter fully described and particularly pointed out in the claim. The annexed drawing and the following description set forth in detail various combinations of ingredients embodying the invention. It is to'be understood however that the scope of the invention is to be limited only by the language of the appended claim interpreted in the light of the prior art.

The coating compound of this invention consists essentially of the following components:

1. Bentonite, a colloidal clay, to add refractory 2. A'hard ground coat frit of the sodium borosillcate type having a maturing temperature of about 1700 F., used as a iiux to create adherence to the ferrous article.

3. Alumina hydrate to increase ,the body of the coating compound, and to add refractoriness. since the flux itself withstands temperatures of 1700 F. for only short periods without burning ,g

sodium silicate may be used having a ratio of 1:2 of sodium oxide to silica, to which enough water is added to form a paste. The sodium silicate serves as a binder to assist in the adherence of the coating to the article being treated.

'I'he proportions of the components 1 to 3 inclusive, that is, the.bentonite,;ilux and alumina hydrate; respectively, were varied within certain limits and the optimum proportions determined under carefully controlled laboratory tests. percentage compositions of these threeV components were plotted on a triangular coordinate diagram which constitutes the singleilgure of the annexed drawing. I'he percentage compositions designated by the points A-,i` tovAl-Il on the triaxial diagram are given in tabular form on the same sheet with the diagram. Within 'the eld studied itwas found lfrom the tests thatthe area around the points AI2 and.A,-i3 on thediagram represented the' proportions whichgave the best results. According tothe tabular record of percentages, ysuch an area may be defined as covering the -following range of compositions:

f lPer cent inclusive Alumina hydrate v .5o to 'zo Flux 20 to 30 Bentonite '5 to 20 Sodium silicate 10 to 30 Additional tests of compositions within the above ranges yindicated thatthe most successful coating compound, judged bythe standards hereinbefore described, is as follows:

Various amounts of the Awater-soluble sodium `silicatepowder used as a'binder, ranging from to 40 per cent of lthecomposition weight have The l beless resistant vto abrasion when dry. In place kof the sodium silicate solutiona water-soluble been used with satisfactory results, but it is preferred that 10 per cent be the minimum weight of powdered sodium silicate to be used since less than 10 per cent causes the coating when dry to be fairly soft and easily abraded.

As a further extended use of the compound of this invention, it should be realized that in the heat treatment of iron or steel parts, which may` include annealing, normalizing, tempering. under conditions which are frequently highly oxidizing, considerable scale formation results. Identincation marks on the castings are often lobliterated due to the heavy scale formed and also due to the subsequent Sandblasting operation afterward which is frequently necessary to remove the scale. Temperatures may vary from 1100 F. to 1800 F. and the time required will be dependent upon the weight of charge and the size of the heat treating furnace.

Castings that had been coated with the compound as given in this invention were heated in an oxidizing'temperature of 1800 F. for three hours, along with uncoated castings.l No scale formation resulted on the coated castings and the coating was easily removed by Sandblasting, leaving a bright iinish. On the untreated castl ings the scale formation was heavy and only removed with difilculty yby Sandblasting.

Thus it `is evident that the range of my novel compound vand its use may-vary considerably. I

. desire therefore to be limited by the scope of the 40v tonite, 5 to 20. per cent; and 

